The present invention relates to a radial roller bearing and, in particular, to a radial roller bearing of a type that, in the end portion of a ring, there is formed a flange portion to be slidingly contactable with the end face of a cylindrical roller.
In a radial roller bearing using a cylindrical roller as a rolling element, in the end portion of a ring, there is formed a flange portion; that is, the end face of the cylindrical roller is slidingly contacted with the present flange portion to thereby control the attitude of the cylindrical roller when it is skewed, or guide the cylindrical roller in the circumferential direction of the ring, or support an axial load applied to the cylindrical roller.
In the radial roller bearing of this type, in case where only the radial load acts on the cylindrical roller and no skew moment occurs in the cylindrical roller, there is raised no problem even when the end face of the cylindrical roller is formed in a straight shape. However, in case where an axial load acts on the cylindrical roller and skew moment or a tilt angle occurs in the cylindrical roller, since the axial load is supported by the contact portion between the cylindrical roller and flange portion, an edge load is generated in the end portion of the contact portion. Due to this edge load, there is applied excessively large surface pressure onto the end portion of the contact portion, with the result that seizure and wear are easy to occur in the end portion of the contact portion. On this account, in a case of that a roller bearing is used under a condition that a certain level of axial load is applied thereon, a tapered roller bearing is chosen in many case. However, the tapered roller bearing needs to adjust the axial clearance between the cup and the cone, which determines the radial clearance and the bearing performances. So, when assembling the bearing into a housing, there is a need to locate the axial position of the cup and the cone precisely. On the other hand, when using cylindrical roller bearing, there is no need to locate the axial position precisely. So, it is very easy to assemble the bearing. The reason why, in case of that the cylindrical roller bearing is used instead of the tapered bearing, it is improved a convenience of assembling. If the axial load capacity of the cylindrical roller bearing is improved, the cylindrical roller bearing ban be used, instead of the tapered roller bearing, under a certain level of axial load. Thus in order to reduce the edge load applied to the cylindrical roller bearing under a certain level of axial load is applied thereon, as shown in FIG. 9, there is known a radial roller bearing in which the end face 13a of a cylindrical roller 13 to be contacted with a flange portion 14 is formed in a spherical shape having a given radius of curvature xcex7.
However, in the radial roller bearing in which the end face of a cylindrical roller is formed in a spherical shape, since the center of the radius of curvature xcex7 lies on its rotation axis 13b of the cylindrical roller 13, as shown in FIG. 10, the contact portion 15 of the cylindrical roller with the flange portion 14 provides almost a circular shape. For this reason, in case where the radius of curvature xcex7 increases as the opening angle xcex4 of the flange portion 14 decreases, the diameter of the contact portion 15 also increases and thus the contact portion 15 swells out of the flange portion 14, so that an edge load is easy to occur.
The present invention aims at eliminating the above-mentioned drawbacks found in the conventional radial roller bearing. Accordingly, it is an object of the invention to provide a radial roller bearing which can restrict occurrence of an edge load in the contact portion between the cylindrical roller and flange portion to thereby be able to enhance the seizure resistance and wear resistance of the contact portion.
In attaining the above object, according to the invention, there is provided a radial roller bearing, comprising: an outer ring; an inner ring; and, a cylindrical roller interposed between the inner ring and the outer ring, the outer and inner rings respectively including flange portions formed in the end portions thereof so as to be opposed to the end face of the cylindrical roller, wherein, in the end face of the cylindrical roller, there is formed a circular-ring-shaped contact portion of the roller end having centers of curvature continuously existing on a circle which lies on a plane parallel to the end face of the cylindrical roller and also the center of the circle is on the rotation axis of the cylindrical roller.
According to the present structure, the contact portions between the above-mentioned contact portion of the roller end formed in the end face of the cylindrical roller and the above-mentioned flange portions each provide an elliptical shape which is narrow in the radial direction and long along the circumferential direction of the flange portion; that is, the elliptical-shaped contact portion is difficult to run off from the flange portion. This can restrict occurrence of an edge load in the contact portion between the cylindrical roller and flange portion, which makes it possible to enhance the seizure resistance and wear resistance of the radial roller bearing.
Now, where the diameter of the cylindrical roller is expressed as Da and the distance from the center of curvature of the contact portion of the roller end to the rotation axis of the cylindrical roller along the radial direction of the cylindrical roller is expressed as "xgr", in the case of "xgr" less than 0.1 Da, an edge load occurs, so that the surface pressure applied to the end portion of the contact portion is larger than the surface pressure applied to the central portion of the contact portion, thereby facilitating the occurrence of seizure. In the case of "xgr" greater than 0.4 Da, an oil film is difficult to be formed in the contact surface to thereby increase the rate of metal contact between the cylindrical roller and flange portion. Therefore, preferably, the distance "xgr" from the center of curvature of the contact portion of the roller end to the rotation axis of the cylindrical roller along the radial direction of the cylindrical roller may be set such that "xgr"=0.1 Da to 0.4 Da.
Also, where the radius of curvature of the contact portion of the roller end is expressed as xcex7, in the case of xcex7 less than 2.0 Da, the film thickness ratio (which is also referred to as an oil film parameter) of the contact portion between the contact portion of the roller end and flange portion is equal to or less than the limit value and thus a rate of metal contact increases. The oil film parameter consists of oil film thickness and composite roughness. The oil film thickness is calculated by EHL theory (Elasto-Hydrodinamic Lubrication Theory). And the composite roughness is calculated with each surface roughness. In the case of xcex7 greater than 20.0 Da, the contact portion between the contact portion of the roller end and flange portion swells out of the flange portion, so that an edge load is generated. Therefore, preferably, the radius of curvature xcex7 may be set such that xcex7=2 Da to 20 Da.
Further, in case where the distance from the center of curvature of the contact portion of the roller end to the rotation axis of the cylindrical roller along the radial direction of the cylindrical roller "xgr" is approximately 0.3 Da, when the composite roughness "sgr" of the contact portion (that is, the roughness that is composed ("sgr"={square root over (("sgr"12+"sgr"22))}) of the surface roughness of the contact portion of the roller end ("sgr"1) and the surface roughness of the flange portion ("sgr"2)) exceeds 0.6 xcexcm, formation of an oil film is very difficult. In the case of "xgr"=0.1 Da or so, when the composite roughness "sgr" of the contact portion exceeds 1 xcexcm, the oil film parameter is equal to or less than 1, formation of the oil film is very difficult. Therefore, preferably, the composite roughness "sgr" of the contact portion may be set such that "sgr"xe2x89xa6xe2x88x9210.4 ("xgr"/Da)2+2.2 ("xgr"/Da)+0.9.